A telemeter allows the measurement of the distance between it and a target. An optical telemeter uses the propagation of light as measuring means, it is composed of an emitter and of a receiver. It emits light directed toward the target and detects a fraction of this light returned by the target. The distance is obtained on the basis of the time required for the light to propagate out to the target and for the light to return to the receiver. The emission is temporally modulated. The emitted light transports this modulation to the target. The target reflects or backscatters this light. A fraction of this returned light transports the modulation to the receiver of the telemeter. Measurement of the time elapsed between the identification of the starting modulation of the telemeter and the identification of the modulation of its return by the receiver makes it possible to calculate the distance between the telemeter and the target on the basis of the speed of propagation of light in the media traversed.
Typically, a telemeter comprises an emission device, comprising an emitter and its optic for shaping the laser beam, a reception device comprising an optic for collecting and focusing on the focal plane laser echoes backscattered by the target, and a processing device for driving the emission and reception and allowing computation of the distance.
The optical echo of the target is converted into an electrical signal by the detector, the electrical signal being superimposed on the noise. The filtered and amplified signal obtained at the end of the detection chain is digitized.
A frame consists of a series of data sampled over the duration counted from the emission of the pulse and over the time of an outbound-return journey corresponding to the maximum distance of use or over the duration corresponding to the distance sub-domain sought. The sampling frequency is chosen so as to optimize the extraction of the signal of the echo from the noise and allow the expected resolution and precision in terms of distance. For example, a sampling frequency of 59.94 MHz would allow a distance increment of 2.5 m.
Various solutions have been implemented for improving the range of a laser telemeter.
A first solution consists in increasing the energy emitted per pulse. But, the increase is limited by constraints of ocular safety and by the increase in volume and in energy consumption of the emission device.
Another solution consists in increasing the surface area of the reception pupil. This solution is, likewise, limited by telemeter bulkiness and weight constraints.
In the case where the dimensions of the target are smaller than the dimensions of the spot made by the laser at the level of the target, only the fraction of light deposited on the target contributes to the telemetry. This fraction is dependent on the quality of the laser beam determining the size of the spot, and on the way the beam is pointed toward the target.
Under ideal conditions, the laser beam is very slightly divergent and perfectly pointed toward the target, all of the emitted light contributes to the telemetry. However, the sighting line is rarely directed toward the part that is most contributory in the telemetry sense, the most contributory part being a zone which returns the largest fraction of the emission by reflection or backscattering toward the reception device. To avoid significant losses of performance, as soon as the sighting line is not directed toward the most contributory part of the target, it is necessary to increase the divergence of the beam, to the detriment of the telemeter's range.
In the case of non-cooperative targets, the laser emission is usually pulsed. A target is cooperative when the target favors the return of the light in the direction of the emitter with the aid of a cubic wedge for example.
In the case of pulsed telemetry, the signal arising from the detector is composed of the noise of the detection chain, of the optical noise collected in the reception field and of the echo of the expected target. When the signal is sufficiently significant, the detection of the moment of arrival of the echo is done by thresholding. Stated otherwise, a target is detected if the intensity of the echo is greater than a threshold fixed beforehand above the level of the noise.
The signal at the moment of the thresholding is the sum of the amplified signal coming from the detection of the echo and the optical and electronic noise. For a target, the signal will have an amplitude varying from one pulse to the next. For a signal in the vicinity of the level of the threshold, the signal will not always exceed the threshold. When the signal is below the threshold there is no detection. If a signal does not at any moment exceed the threshold, the echo is absent or too weak.
The observation can be done from the start of the pulse over a duration corresponding to the maximum distance sought, for example 533 μs for a maximum distance of 80 km. The observation can also be done over a duration corresponding to a distance sub-domain, for example over a duration corresponding to the sub-domain lying between 40 and 50 km.
Another possibility for improving the probability of target detection over a given time interval is to increase the pulse repetition rate.
To improve the detection of the echoes of the target in relation to noise, it is possible to combine the detection signals subsequent to several pulses. The combining of several detection signals can be undertaken according to a post-integration method. This procedure is old, it has been implemented with analog methods but it is still in vogue in the digital age.
Post-integration processing is a way of combining the frames of signals detected subsequent to each pulse.
For a given telemeter, the post-integration step makes it possible to improve the gains appreciably when the distance between the telemeter and the target is sufficiently stable over the duration of the measurement.
In the case where the telemeter is properly pointed at the target the probability of the presence of the echo of the target in each frame is 1. If the distance between the telemeter and the target varies little in the course of the post-integration phase, at each distance increment, the data of frames are added up. The expected signal S is added up linearly, it is therefore proportional to N, N being the number of frames,
            ∑      N        ⁢    S    ∝      N    .  On account of its nature, the detection noise B is summed quadratically, the amplitude of the noise is proportional to the square root of the number of frames,
            ∑      N        ⁢    B    ∝            N        .  The ratio of the intensity of the expected signal to the intensity of the noise will be proportional to the square root of the number of frames,
                    Σ        N            ⁢      S                      Σ        N            ⁢      B        ∝            N        .  
Stated otherwise, for a post-integration step on N frames having a probability of presence of the echo of the target in a frame of 1, the ratio of the intensity of the signal to the intensity of the noise S/B is proportional to √{square root over (N)}.
During a difficult pursuit of a mobile target, notably when the dimensions of the target are smaller than the dimensions of the spot of the laser at the level of the target, certain frames do not contain any information relating to the presence of an echo of the target. The post-integration applied to all the frames does not have the expected effectiveness. Frames which contain only noise are thus added to the frames which also contain an echo of the target. The probability of an echo of the target in a frame therefore directly affects the gain expected by the post-integration step.
When the probability of presence of the target on the spot of the laser at its level is 1/a, that is to say only one frame out of a frames comprises an echo of the target, the ratio of the intensity of the signal to the intensity of the noise S/B is proportional to
            N        a    .Therefore a2·N frames are necessary for the same ratio of the intensity of the signal to the intensity of the noise √{square root over (N)} as that obtained in n frames when the probability of presence of the echo is 1. To obtain for example a gain of 10(√N=10) subsequent to the post-integration step, this requires the summation of 100 frames. If the probability of presence of the echo of the target in a frame is ½ then a gain of 10 will be obtained by summing 22·100=400 frames.
An aim of the invention is to improve the performance of a telemeter using a step of post-integration, using the invention previously described in patent application EP 2364455. This patent application EP 2364455 proposes a telemetry reception device capable of detecting temporally and spatially the echo provided by the target illuminated by the laser pulse.
The temporal detection allows the distance to be measured by measuring the time of flight of the pulse, it can be done by means of one or more detectors.
The spatial detection can be obtained by means of one or more detectors. This detection, on the basis of one or more pulses, makes it possible to label the direction from which the maximum of light backscattered by the target comes or the absence of target. This maximum of light results from the interaction of the target with the spot of the laser pulse. It is thus possible to recenter the direction of emission so as to maximize the effectiveness of the telemeter.
The passband required for the temporal detection is very large in comparison to that of the spatial detection, thus increasing the noise of the temporal detection chain. Consequently, the spatial detection is much more sensitive than the temporal detection.